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However, the value of crack closure stress ( I i SB cc sb ) and crack initiation stress ( I i SB ci sb ) will be clearly changed by small fluctuations of Poisson's ratio. If the stress level is higher than the crack damage stress ( I i SB cd sb ) but lower than peak strength ( I i SB f sb ), cracks will propagate continuously and coalescence to reach macroscopic failure. 6 Curves of lateral strain difference for a siliceous siltstone and b limestone Moving Point Regression Method (MPR) Eberhardt et al. ([4]) proposed a new method to obtain the crack closure stress ( I i SB cc sb ) and the crack initiation stress ( I i SB ci sb ). If the stress level is higher than crack initiation stress ( I i SB ci sb ) but lower than crack damage stress ( I i SB cd sb ), cracks randomly initiated but they will cease at a certain length without coalescence. [Extracted from the article]

• It is improper to determine the failure nature of macro-crack by using the proportion of micro-cracks (tensile ⧹ shear). • Moment tensor inversion can indicate the forces applied on failure sources and distinguish the nature of primary and secondary cracks. • The failure modes of secondary cracks are complicated, changing from shear to tensile as the cracks propagate. Cracking processes have been extensively studied in brittle rock and rock-like materials. For a better understanding of failure processes, moment tensor is used to distinguish the nature of primary cracks and secondary cracks in a single-flaw-contained specimen based on the bonded-particle model, which can indicate the force applying on the failure source. The results reveal that the primary cracks solely consist of micro-tensile cracks and the failure nature is tensile, while the secondary cracks are composed of both micro-shear cracks and micro-tensile cracks and the failure mechanisms are complicated. The failure mechanisms of acoustic emission (AE) events of the first part of secondary cracks, which initiate from the tips of the pre-existing flaw, are mainly "implosional" and "shear", implying that the unbalance forces applied on the failure source are predominately implosional and shear components after breakage. As the secondary cracks propagate farther, the failure nature of AE event changes to "tensile" mode finally. [ABSTRACT FROM AUTHOR]

Abstract To better investigate hydraulic fracturing in tight reservoirs, interaction modes between a hydraulic fracture and a weak plane are studied with acoustic emission (AE) based on moment tensor under different differential stresses (Δσ) and approaching angles (β). Failure nature of hydraulic fracture is mainly tensile based on moment tensors except that the hydraulic fracture propagates into weak plane. At β = 30°, the hydraulic fracture is arrested by the weak plane. The failure nature of weak plane changes from 'Shear' (Δσ = 10, 7 and 5 MPa) to 'Tensile' (Δσ = 3 MPa). At β = 60°, the hydraulic fracture branches into weak plane as crossing, the nature of which is 'Shear' as differential stress decreases from 10 MPa to 5 MPa. At β = 90°, the hydraulic fracture crosses the weak plane perpendicularly. [ABSTRACT FROM AUTHOR]

To better investigate the cracking processes under uniaxial compression, a numerical approach using bonded-particle model (BPM) was adopted to simulate the loading processes and AE events of limestone. To validate the model, a physical experiment with acoustic emission (AE) monitoring was performed. It revealed that the AE parameters in the BPM including crack number and AE event were generally comparable with the AE count and AE hit in the experiments. The b-value of AE does not accurately evaluate the degree of damage in rocks. But it can indicate the different states of damage during cracking processes. [ABSTRACT FROM AUTHOR]

To investigate the acoustic emission characteristic of rock under different compressive loading rates, a numerical approach using the bonded-particle model (BPM) is adopted in the present study. It reveals that under uniaxial compressive test, the physical mechanism and distribution of AE events recorded by the BPM are generally consistent with the phenomena observed from the laboratory experiments. The total crack number, tensile crack number, shear crack number and AE event number increase as the loading rate increases. The increase of loading rates from 0.005 m/s to 0.6 m/s in the BPM does not have obvious influence on the distribution of AE events. [ABSTRACT FROM AUTHOR]

• The AE characteristics of specimens containing hole-like flaw are reproduced by the BPM. • The fracture evolution and failure mode of these specimens with hole-like flaw of different shapes are revealed. • The failure mechanism of fractures initiating from around the hole-like flaw is discussed. For a better understanding of acoustic emission (AE) characteristics and failure mechanism of specimens containing hole-like flaw, a series of uniaxial compression tests are carried out by using the bonded-particle model (BPM). In the BPM, the AE simulation combined with moment tensor inversion is adopted. The results indicate that the stress drop in the stress–strain curve corresponds to a sudden increase of AE counts, which results from the propagation and coalescence of micro-cracks. The shape of hole-like flaw does not have a significant influence on the magnitude distribution of AE event. The b-value of three models is similar, i.e. 1.68, 1.83, and 1.75 corresponding to circular, square, and horseshoe flaw respectively. The forces concentration at upper and bottom of the hole-like flaw is dominated by tensile, and the fracture path only contains micro-tensile cracks. While the forces acting on lateral sides of the hole-like flaw are dominated by compressive. The fracture path contains both micro-tensile and micro-shear cracks and the failure nature is shear and compaction, although the number of micro-tensile crack is predominant. [ABSTRACT FROM AUTHOR]

To have a better understanding of initiation and propagation of hydraulic fracture in transversely isotropic reservoir, a bonded-particle model (BPM) coupling fluid-mechanical was carried out to investigate the fracture networks in tight sandstone with different dip angles. Acoustic emission technique is applied in hydraulic fracturing modeling to monitor crack initiation and propagation. The results indicate that the complex fracture network is more prone to form under low differential stress. The magnitude of AE events of the hydraulic fracture along the bedding planes is lower, compared to those propagating at the angle with respect to the direction of the bedding plane. The interaction mode between the hydraulic fracture and bedding planes can be categorized into 4 types: (1) fracture network (location at the borehole) + multi-branch fractures; (2) fracture network (location at the vicinity of the hydraulic fractures) + multi-branch fractures; (3) multi-branch fractures; (4) a single fracture. • Fluid-mechanical model was used to study behavior of hydraulic fracture. • The AE magnitude of hydraulic fractures in transversely isotropic rock is studied. • The interaction mode between hydraulic fracture and bedding plane was categorized. [ABSTRACT FROM AUTHOR]

To investigate the crack propagation and failure mechanisms of brittle materials in the Brazilian tests, an acoustic emission (AE) simulation was implemented in the Discrete Element Method. AE location and magnitude were monitored during the whole process of the simulation to detailedly observe crack initiation and associated AE evolution. Moment tensors were directly calculated by the forces and motions of the particles and decomposed into isotropic and deviatoric parts. According to the decomposition results of moment tensors, AE events (microcracks) can be classified into explosive (tensile), shear and implosive sources. Explosive sources were found to dominate both the total number and energy emission, followed by shear sources and finally implosive sources. AE distributions of different AE types were compared with the stress distribution to explore the relation between failure evolution and intrinsic stress state of the Brazilian test. Furthermore, the approach was applied to the compression test with a single flaw. Comparisons with experimental results indicated that the AE simulation can overcome the shortcoming of signal missing near the peak load in experiments and is approved to be an effective and accurate way to study rock fracture mechanism. [ABSTRACT FROM AUTHOR]